North Korea said Monday its ruling party would hold a special conference on April 11, bolstering the power of its young leader just before a major anniversary and the planned launch of a long-range rocket.

Preparations for blast-off sometime between April 12-16 are further advanced than previously believed, a US specialist website reported, citing a new satellite photograph.

The North says its rocket will put a peaceful satellite into orbit but the United States, South Korea and other nations see it as a pretext for a long-range missile test banned by the United Nations.

The 38 North website (38north.org) said a March 28 photo of the launch site at Tongchang-ri in the country's far northwest appeared to show a mobile radar tracking system atop a ridge at the end of a new dirt road.

It said the image also shows previously empty areas filled with rows of what are probably empty fuel and oxidiser tanks, apparently dumped after their contents had been transferred in preparation for launch.

The North is preparing mass celebrations to mark the 100th anniversary on April 15 of the birth of Kim Il-Sung, the country's first and "eternal" president and founder of the dynasty which has ruled uninterrupted since 1948.

A successful satellite launch would burnish the image of his grandson Kim Jong-Un as he seeks to establish his credentials as a strong leader.

The party meeting is expected to wrap up the power transfer to Jong-Un, proclaimed "great successor" after the death of his father Kim Jong-Il last December.

So far Jong-un, aged in his late 20s, has been formally appointed to only one of his late father's posts -- supreme commander of the 1.2-million-strong military, the world's fourth largest.

The Workers' Party of Korea meeting is likely to appoint Jong-Un to the post of party general secretary previously held by his father.

Party committee meetings in the military, provinces, cities and counties elected Jong-Un as one of the conference delegates, "reflecting the unanimous will and desire of all the party members, service personnel and people", the official news agency said.

Separately, the North will convene an annual session of its rubber-stamp parliament on April 13. Legislators have the power to appoint a chairman of the National Defence Commission, the top decision-making body.

Kim Jong-Il previously chaired the commission. It was unclear whether his son would take over the post.

The North disclosed its satellite plan just over two weeks after announcing a deal with the United States.

It had agreed to suspend operations at its Yongbyon uranium enrichment plant, and impose a moratorium on long-range missile tests and nuclear tests, in return for 240,000 tonnes of US food aid.

Washington said last week it was suspending plans to start food deliveries.

Pyongyang, which insists that a satellite launch is not a missile test, criticised the US move Saturday as an "over-reaction" that would kill off the February 29 agreement.

Several foreign leaders at a nuclear summit in Seoul last week reportedly criticised the launch as a waste of money when the North is struggling to feed its people.

An unidentified South Korean military official quoted by Yonhap news agency estimated the launch cost at $850 million -- $400 million for construction of the launch site, $300 million for the rocket and $150 million for the payload.

This could buy 2.5 million tonnes of Chinese corn and feed the North's entire population for a year, the official said, adding Pyongyang is expected separately to spend $2 billion on celebrating the birth anniversary.

Southeast Asian nations voiced concern Monday about the launch plan.

"There is a real concern on the development in the Korean peninsula," said ASEAN chief Surin Pitsuwan after foreign ministers of the 10-member Association of Southeast Asian Nations met in the Cambodian capital.

"Instability up there could lead to diminishing confidence in the region as a whole."